Adaptive braking system

ABSTRACT

An adaptive braking system for an automotive vehicle is disclosed which includes a single comparator which compares a deceleration signal representative of wheel deceleration with a pair of reference levels. When wheel deceleration exceeds the first reference level, the comparator generates an output signal which activates an adaptive braking valve and which also activates a timer which produces an output signal for a predetermined time period after activation of the timer. The output signal from the timer switches the comparator to the second reference level which is lower than the first reference level, maintaining the adaptive braking valve on as long as wheel deceleration exceeds the second reference level. When the timer period expires, the comparator is switched back to the first reference level, to turn off the adaptive braking valve if wheel deceleration is less than the first reference level. A mercury switch which is responsive to vehicle deceleration is provided. When the mercury switch senses vehicle braking on a low coefficient surface both of the comparator reference levels are changed by a predetermined amount, although the difference between the reference levels remains the same.

Needham et al.

[ Nov, 12, 1974 ADAPTIVE BRAKING SYSTEM Inventors: James M. Needham; Delbert R.

Elliott, both of South Bend, 1nd.

Assignee: The Bendix Corporation, South Bend, 1nd.

Filed: June 15, 1972 Appl. No.: 263,518

U.S. Cl. 303/21 CG, 188/181 A Int. Cl B60t 8/12 Field of Search 188/181 A; 303/20, 21 AU;

[56] References Cited UNITED STATES PATENTS Primary Examiner-Milton Buchler Assistant ExaminerStephen G. Kunin Attorney, Agent, or Firm1(en C, Decker; William N. Antonis oem'vnrivz TH c H FILTER M p l I [Q I8 20 [57] ABSTRACT An adaptive braking system for an automotive vehicle is disclosed which includes a single comparator which compares a deceleration signal representative of wheel deceleration with a pair of reference levels. When wheel deceleration exceeds the first reference level, the comparator generates an output signal which activates an adaptive braking valve and which also activates a timer which produces an output signal for a predetermined time period after activation of the timer. The output signal from the timer switches the comparator to the second reference level which is lower than the first reference level, maintaining the adaptive braking valve on as long as wheel deceleration exceeds the second reference level. When the timer period expires, the comparator is switched back to the first reference level, to turn off the adaptive braking valve if wheel deceleration is less than the first reference level. A mercury switch which is responsive to vehicle deceleration is provided. When the mercury switch senses vehicle braking on a low coefficient surface both of the comparator reference levels are changed by a predetermined amount, although the difference between the reference levels remains the same.

5 Claims, 4 Drawing Figures 55 05cm! P30 66 VALVE TIMER URN/ER DECAY BRAKE VHLVE I SIGNAL T T 29 o ISOLATION VALVE sum 10F 2 PATENTEDHuv 12 I374 ADAPTIVE BRAKING SYSTEM BACKGROUND OF THE INVENTION This invention relates to an adaptive braking system for an automotive vehicle.

Conventional adaptive braking systems compare wheel deceleration with a fixed reference level. When the wheel decelerates past the reference level, an adaptive braking cycle is initiated. To prevent premature termination of cycle, wheel acceleration through a higher reference level is required before the adaptive braking cycle is terminated. The present invention uses a timer activated by the signal which initiates the adaptive braking cycle to change the reference level of the comparator to the higher value. Therefore, the adaptive braking cycle is terminated when wheel acceleration exceeds thehigher reference level, or upon expiration of the timer period during wheel acceleration exceeding the lower reference level but not exceeding the higher reference level.

It is also desirable to change the adaptive braking circuitry during vehicle braking on a low coefficient surface. Therefore, a vehicle deceleration responsive mercury switch is provided which changes both of the reference levels when the vehicle is braked on a low coefficient surface. Although the magnitudes of both of the reference levels are changed, the difference between the reference levels may remain the same, or may be different dependent upon the needs of the adaptive braking system.

SUMMARY OF THE INVENTION Therefore, an important object of our invention is to provide an adaptive braking system for an automotive vehicle in which deceleration reference levels are altered by a vehicle deceleration responsive switch when the vehicle is operated on a-low coefficient surface.

Another important object of our invention is to provide an adaptive braking system having a timing mechanism producing an output signal for a predetermined mechanism activated by initiation of an adaptive braking cycle.

DESCRIPTION OF. THE DRAWING FIG. 1 is a graphical representation of wheel acceleration vs. time and wheel speed vs. time during operation of an adaptive braking system made pursuant to the teachings of our present invention illustrating a cycle in which the speed recovery mode is not used;

FIG. 2 is a graphical representation similar to FIG. 1 but illustrating a cycle in which the speed recovery mode of operation is used;

FIG. 3 is a block diagram of an adaptive braking system made pursuant to the teachings of our present invention; and

F IG. 4 is a more-detailed illustration of the block diagram illustrated in FIG. 3 which shows circuit details of many of the more important components of our system.

DETAILED DESCRIPTION Referring now to FIG. 3 of the drawings, an adaptive braking system generally indicated by the numeral 10 includes an electromagnetic wheel speed sensor 12 which is responsive to the rotational speed of a controlled wheel of the vehicle 14. The wheel speed sensor 12 is of the'conventional electromagnetic type which produces an alternating voltage output, its varying in proportion to the angular velocity of the wheel 14. Since the electromagnetic wheel speed sensor l2 is of a completely conventional type, it will not be further described herein. The alternating voltage produced by the wheel speed sensor 12 is converted by a tachometer 16 to a pulsed output the frequency of which is propor tional to that of the wheel speed sensor input. The pulsed output produced by the tachometer 16 is processed by a conventional filter 18, so the output of the filter 18 is a voltage which is proportional to the angular velocity of the wheel 14. The output signal of the filter 18 is differentiated by a derivative amplifier 20, of a conventional design well known to those skilled in the art. The derivative amplifier 20 includes an inverter, so that positive values of the output voltage of the amplifier 20 represent wheel deceleration and negative values represent wheel acceleration. The output of the responding to the G acceleration level as illustrated in FIG. 1. Whenever the input to the comparator 22 exceeds this G voltage level,'the comparator 22 produces a control signal which is transmitted to the reset input 23 of a flip flop 24.

The flip flop 24 is of a conventional type having the following truth table:

In other words, if the signals to both the reset input 23 and the set input 25 of the flip flop 24 have logic lev-. els of 0, the flip flop remains in its previously existing state. However, if a signal-having a logic level of l is transmitted to the reset input 23, the Q outputwill be at a logic level of 0. Similarly, if the signal transmitted to the reset input 23 has a logic level of 0 while the signal transmitted to the set input 25 has .a logic level'of l, the 0 output will have a logic level of 1. As .will be discussed in detail hereinafter, the signal transmitted to the set input 25 normally has a logic level of l.

. When the control signal is received at the reset input 23 of the flip flop 24, the positive output or Q of the latter changes states, from a logic level of l to a logic level 'of 0. The Q output of the flip flop 24 remaining at a O logic level as long as an input signal is received at either input 23 or 25 and is inverted by an inverter 26. Thereof the inputs of an'AND gate 28, the-other input of the v latter being connected to the stoplight switch of the vehicle so that the AND gate 28 produces an output signal only when the Q output of the flip flop 24 is at a logic level of and the brakes of the vehicle have been applied. The positive output of the AND gate 28 actuates the decay valve driver circuitry 30, which is of conventional construction well known to those skilled in the art and which consist primarily of an amplifier which actuates the solenoid-actuated decay valve represented by the coil 32 in FIG. 3. The output of decay valve driver also actuates the isolation valve timer 34, the output of which is transmitted to one of the inputs of AND gate 36, the other input being connected to the aforementioned brake light switch. The output of AND gate 36 is connected to an isolation valve driver circuit 38, which is of similar construction to the decay valve driver circuitry 30. The isolation valve driver circuitry 38 controls the operation of the solenoid-actuated isolation valve which is represented in FIG. 3 by the coil 40. The valves 32 and 40 are more completely disclosed in copending US. Patent Application Ser. No. l79,9l9, filed Sept. 13, 1971, owned by the assignee of the present invention and incorporated herein by refer ence. The isolation valve timer 34 is actuated by the decay valve driver 30 and merely insures that the isolation valve 40 will not be turned off before the decay valve 32 is turned off.

A positive output from the decay valve driver 30 also actuates a timer mechanism generally indicated by the numeral 29 in FIG. 3. The timer mechanism 29 is of conventional construction and produces an output signal for a predetermined time period after a positive input signal is received. The timer is effective to switch the comparator 22 from theG reference level to a much higher G reference level once the comparator has generated an output control signal to the reset input 23 of the flip flop 24. Therefore, referring again to FIGS. 1 and 2, while the decay valve driver 30 will be turned on when the wheel 14 decelerates to the G level, it will not be turned off until the wheel acceleration exceeds an acceleration level represented by the G level in FlG. 1. If the wheel acceleration does not reach the G acceleration level before the period of timer 29 expires, the comparator 22 will be switched back to the G reference level, so that the valve 32 will be turned off if the wheel is accelerating at a level above the G 1 reference level. A mercury switch generally indicated by the numeral 31 also controls the G lG comparator and changes the reference level ofboth the G and G reference levels if the vehicle is decelerating below a predetermined rate. Since the rate of vehicle deceleration is roughly indicative of the coefficient of friction between wheels of the vehicle and the surface across which they are traversing, the mercury switch 30 is effective to decrease the G and G reference levels when the vehicle traversing a surface having a low coefficient of friction. This is most desirable since higher wheel decelerations may be tolerated on a high coefficient surface before the wheel locks to throw the vehicle into a dangerous skid. Details of the construction of the mercury switch 30 are disclosed in U.S.'Pat. No. 3,525,553 owned by the assignee of the present invention and incorporated herein by reference. The manner in which the mercury switch changes the G and G reference levels will be more completely disclosed hereinafter.

The output of the inverter 26 is also transmitted to the enable input 106 of a speed memory 33. A pair of resistors and 37 define a voltage dividing network and are connectedbetween the output of the filter 18 and the electrical ground at 39. The input to the speed memory 33 is connected between the resistors 35 and 37 and therefore transmits a voltage to the speed memory 33 which is proportional to a predetermined percentage of the rotational speed of the wheel 14, the percentage being controlled by the relative values of the resistors 35 and 37. In practice, it has been found desirable to memorize a voltage representing approximately 80 percent of the speed of the wheel 14. Therefore, when a positive signal is received at the enable input 106 of the speed memory 33, the memory 33 memorizes a value corresponding to the predetermined percentage of the then existing speed of the wheel 14.

The output of the speed memory 33 is transmitted to a speed comparator generally indicated by the numeral 41, and the other input of the comparator 41 is connected to the output of the filter 18, which varies directly with the speed of the wheel 14. The output signal generated by the comparator 41 normally has a logic level of l, but switches to transmit a signal'having a O logic level to the set input 25 of the flip flop 24 whenever the speed received at the terminal 44 of the comparator 41- is less than the memorized value stored in speed memory 33 connected to the input terminal 46 of speed comparator 41, it being realized that the value stored in the speed memory 33 is equal to the predetermined percentage of the wheel speed existing at the time the decay cycle was initiated. Therefore, as long as the speed of the wheel 14 is less than the predetermined percentage of the wheel speed existing at the time the decay cycle was initiated, the Q output of the flip flop 24 will remain at a logic level of 0, since an input signal received at both the set terminal 25 and the reset input 23 of the flip flop 24 have 0 logic levels.

Therefore, since the Q output of flip flop 24 remains at a 0 level, and since this output is inverted by inverter 26, the decay valve 32 will remain on until the wheel speed exceeds the memorized value stored in the speed memory 33.

It has been found desirable to provide a relatively slow decay of the memorized value stored in the speed memory 33, so that the longer the decay valve remains on, a proportionally lower speed of the wheel 14 is required in order to turn the decay valve 32 off. This decay is necessary since under some conditions of operation of the adaptive braking system 10 the vehicle may decelerate sufficiently during the first cycle of the adaptive braking system after a brake application is initiated that the memorized speed would never be reattained, thereby leaving the decay valve turned on. Details of the operation and construction of this bleed from the decay memory will be described in detail hereinafter.

Referring now to FIG. 4 of the drawings, the comparator 22 includes a pair of transistors 48, 50. Transistor 48 is connected between an electrical ground as at 52 and through a resistor 56 to a supply line 54 which is maintained at a constant voltage by a power supply (not shown). The base of the transistor 48 is connected to the output of the derivative amplifier 20. Transistor 50 is connected to the supply line 54 and through resistor 56 to the reset input 23 of flip flop 24. The voltage dividing network consisting of resistor 58, 60, 62 and 64 which are connected between the supply line 54 and the electrical ground as at 66 establish the bias on the base of the transistor 50. A transistor 68 is connected around the resistor 58, and the bias on the base of'the transistor 68 is controlled by the output of the timer 29. Since normally there is no output from the timer 29, and normally the transistor 68 is conductive, the resistor 58 is normally switched out of the voltage dividing network establishing the bias on the base of the transistor 50. Resistor 64 is connected in series with a transistor 70, and the transistor 70 and resistor 64 are connected in parallel with the resistor-62. The bias on the base of the transistor 70 is also controlled by the mercury switch 31, which is connected between the voltage supply line 54 and ground. Since normally the mercury switch is open and the transistor 70.is conducting, the resistor 64 is switched in parallel with resistor 62. However, when the vehicle is traversing a road surface which has a relatively high coefficient of friction when a brake application is initiated, the resultant rather rapid vehicle deceleration closes the mercury switch 31 to change the bias on transistor 70 to switch the resistor 64 out of the voltage dividing network, to thereby raise the bias on the base of transistor 50.

Normally, transistor 48 is turned on, so that the resistor 56 is connected directly to the ground as at 52,

thereby suppressing all electrical signals through the transistor 50 to the set input 23 of flip flop 24. However, when the output of the derivative amplifier 20, which is proportional to the deceleration of the controlled wheel 14, exceeds the bias established on the base of transistor 50 by the voltage dividing network, transistor 48 will be turned off and transistor 50 will be turned on, to thereby transmit an electrical signal to the reset input 23 of flip flop 24 which actuates the decay valve driver mechanism as mentioned hereinabove. It must be remembered that normally the bias on the base of transistor 50 is established by the resistors 60, 62 and 64. This represents the G level shown on FIGS. 1 and 2. After the reset input 23 of flip flop 24 is actuated, the timer 29 is also actuated as described hereinabove, thereby turning off the transistor 68 to switch the resistor 58 into the voltage dividing network to thereby lower the bias on the base of the transistor 50. When the transistor 68 is switched,-the bias on the base of transistor 50 is changed to correspond with the G ref-' erence level on FIG. 1. Therefore, while a relatively low deceleration is required to initially render transistor 50 conductive to reset the flip flop 24, a lower voltage output of the derivative amplifier is required to turn the transistor 50 back off. Similarly, when the vehicle is traversing the surface having a relatively high coefficient of friction when the brakes are applied, the mercury switch 31 will be closed, thereby switching the transistor 70 off to switch resistor 64 out of the voltage dividing network, thereby lowering the bias on the base of the transistor 50. Therefore, since the resistor 60 or the combination of resistors 58 and 60 are connected in series with the resistor 62, both theG and G reference levels will be raised by operation of the mercury switch, but the difference between the G and G reference levels will remain the same.

Referring now to the speed memory circuit, generally indicated by the numeral 33, a speed memory amplifier 71 has an input 72 which is connected between the resistors 35 and 37 which comprise the aforementioned voltage dividing network regulating speed memory 33 so that the voltage between the resistors is proportional to the aforementioned predetermined percentage of wheel speed. The amplifier 71 is a conventional operational amplifier with a gain of one, so that the detail of the amplifier 71 need not-be disclosed in detail. Therefore, the voltage appearing at the output terminals 74 of the speed memory 71 is also proportional to the predetermined percentage of speed of the wheel 14. Capacitor 76 is connected between another input 78 of the amplifier 71 and the ground line 80. A relatively small resistor 82 and a transistor 84 are connected in parallel with the capacitor 76. The bias on the base of the transistor 84 is established by the Q output of the flip flop 24. Therefore, when the output signal from the flip flop 24 is at a high voltage level, which is its normal state, the transistor 84 will be turned on thereby providing a discharge path through the relatively small resistor 82 for the capacitor 76. As long as the wheel 14 is rotating, the instantaneous voltage across the capacitor 76 will be proportional to the predetermined per- I centage of wheel speed established by the resistors 35 and 37. However, when the inverted output of flip flop 24 switches to a logic level of 0, the resistor 82 will be I switched out of the circuit and therefore the voltage then existing across the capacitor 76 will be stored. As pointed out hereinabove, this-voltage corresponds to the predetermined percentage of wheel speed at the time the decay valve is actuated. An isolating diode 86 is provided to isolate the capacitor 76 so that the charge existing across the latter when the transistor 84 is turned off cannot bleed back through the speed memory amplifier 71. A resistor 88, of a relatively large value, is connected in parallel with the capacitor 76 and bleeds the charge from the latter at a predetermined rate. This rate is equal to the decay rate of the value stored in the speed memory 33 as mentioned hereinabove, and, of course, is dependent upon the relative value of the resistor 88 and the capacitor 76.

One input of the comparator 41 is connected to the capacitor 76 and a second input of the comparator 41 is connected to the output of the filter 18, where it transmits a signal to the comparator 41 which is proportional to the instantaneous speed of the controlled wheel 14 as pointed out hereinabove. The output signal of the comparator 41 normally has a logic level of 1. Whenever the signal which is representative of the instantaneous wheel speed drops below the voltage across the capacitor 76, the comparator 41 switches to generate an output signal having a O logic level which is transmitted to the set input 25 of the flip flop 24 where it maintains the Q output of flip flop 24 at a 0 logic level as pointed out hereinabove. The decay valve, therefore, remains on as long as the O logic level signal is generated by the comparator 41.

MODE OF OPERATION mined percentage of true wheel speed which is stored in the speed memory 33 during an adaptive braking cycle. Referring now particularlyto FIGS. 1 and 2 of the drawings, an antiskid cycle is initiated whenever the wheel deceleration as illustrated by the uppermost graph of FIGS. 1 and 2, exceeds the G reference level. In the ensuing discussion, it will be assumed that the vehicle is operating on a low coefficient surface so that the mercury switch will be open to insure operation of the antiskid system whenever wheel deceleration exceeds the G reference level and to terminate the signal to the reset input 23 of flip flop 24 whenever the wheel acceleration exceeds the G reference level. Of course, if the vehicle is operating on a high coefficient surface, the antiskid system works in exactly the same way except that a much higher G deceleration level is required, as represented by the G acceleration on FIGS. 1 and 2, and a correspondingly higher G deceleration level is required in order to terminate the signal to the reset input 23 of flip flop 24 as represented by G reference level in FIGS. 1 and 2. As pointed out hereinabove, when wheel deceleration exceeds the G, reference level, a signal is transmitted to the reset input 23 of flip flop 24 which actuates the decay valve 32. As can be seen from the lower graph on FIGS. 1 and 2, the corresponding wheel speed when the G deceleration level is reached is represented by numeral 92. When the decay valve is actuated, a value equal to, for example 80 percent of the wheel speed existing at that instant is memorized in the memory circuit 33. This value corresponds to the point 94 on the lower curves of FIGS. 1 and 2. As also was pointed out hereinabove,

the value stored in the speed memory 33 is decayed at a predetermined rate. This rate is represented by the line 96 in FIGS. 1 and 2, the slope of which is equal to the decay rate of the value stored in memory 33.

When the decay valve is actuated when a G deceleration reference level is attained, the wheel continues to decelerate until it reaches a peak represented by the numeral 98 on the upper graphs of FIGS. 1 and 2 due to time lag in the system and then it begins to accelerate until it reaches a G acceleration level as illustrated in the upper graphs of FIGS. 1 and 2. If wheel speed follows the solid curve on the lower graph on FIG. 1, the wheel speed corresponding to the G acceleration level is represented by point 100 on FIG. 1. Since the point 100 is higher than the line 96 corresponding to the memorized value of instantaneous wheel speed, the decay valve closes and the adaptive braking cycle terminates. However, under some braking conditions, the wheel speed will follow the solid line illustrated in the lower graph of FIG. 2 so that the wheel decelerates to a much lower speed before the adaptive braking cycle can permit the wheel to reaccelerate. Such a cycle is represented by the solid line in the lower graph of FIG.

2. As can be seen from this graph, the instantaneous wheel speed when the G acceleration level is reached, represented by point 102 in FIG. 2 is much closer to 0 wheel speed, than it was when the G acceleration level was attained in FIG. 1. Since if the brakes were reapplied when the wheel was rotating this slowly the wheel could immediately lock up before the antiskid system could react, it is therefore desirable to maintain the decay valve on until the wheel speed recovers. This is achieved by the speed memory 33, the comparator 41, and the set input of flip flop 24, which prevents termination of the signal to the decay valve until the wheel speed accelerates above the line 96, as represented by a point 104 in FIG. 2. When the speed of the wheel exceeds the memorized value stored in the speed memory 33, as represented by the point 104 in FIG. 2, the decay valve is turned off and the adaptive braking cycle is terminated at that time.

In conclusion, an adaptive braking cycle is always initiated by wheel deceleration which exceeds the G acrences. If the wheel accelerates past the G acceleration reference level, and the speed of the wheel exceeds the value stored in the speed memory 33, the cycle is terminated. The cycle is also terminated if the time period of the timer 29 has expired and wheel acceleration exceeds the G reference level but is less than the G reference level and the wheel speed exceeds the value stored in the speed memory 33. The third manner in which the adaptive braking cycle may be terminated is the condition in which the wheel speed remains below the value stored in the speed memory circuit 33 when the wheel acceleration exceeds the G reference level. When this occurs, the adaptive braking cycle is terminated only when the wheel speed accelerates above the value stored in the speed memory 33.

We claim:

1. In a wheeled vehicle having a brake for braking a corresponding wheel, an adaptive braking system for controlling said brake to prevent skidding of said corresponding wheel comprising:

electrically operated valve means for relieving braking pressure in said brake controlling said corresponding wheel to prevent skidding of the latter; and

control means for actuating said valve means including first means for generating a speed signal proportional to the angular velocity of said corresponding wheel, second means for differentiating said speed signal to generate an acceleration signal proportional to acceleration and deceleration of said corresponding wheel, means for generating first and second bias signals having predetermined values which are independent of fluctuations in said speed signal, said acceleration and deceleration signals, and the rate of deceleration of the vehicle, said means for generating said first and second .bias signals being responsive to actuation of said valve means to change the values of said first and second bias signals by predetermined amounts which are independent of fluctuations in said speed and acceleration signals and of deceleration of said vehicle, switching means activating said valve means when said acceleration and deceleration signal drops below the value of the corresponding bias signal and terminating said valve means when said acceleration and deceleration signal exceeds the value of the then corresponding bias signal, bias means switchable from a first state to transmit said first bias signal to said switching means to a second state transmitting said second bias signal to said switching means, and means responsive to vehicle deceleration for switching said bias means between said first and second states.

2. The invention of claim 1:

said control means including mechanism generating a timer signal for a predetermined time period after actuation of said valve means, said means for generating said first and'second bias signals being responsive to said timer signal for changing the value of said bias signals by said predetermined amounts.

3. The invention of claim 1:

said means responsive to the vehicle deceleration changing the first and second values of the bias signal by an equal predetennined amount which is independent of said speed and acceleration signals and of deceleration of said vehicle.

4. The invention of claim 1:

said means for generating first and second bias signal including a voltage dividing network comprising first resistance means connected in series with second resistance means, said first resistance means including a pair of resistors connected in parallel, said vehicle deceleration responsive switch means switching one of said pair of resistors into said network during vehicle braking on a low coefficient surface and switching said one of said pair of resismeans being responsive to said timer signal. 

1. In a wheeled vehicle having a brake for braking a corresponding wheel, an adaptive braking system for controlling said brake to prevent skidding of said corresponding wheel comprising: electrically operated valve means for relieving braking pressure in said brake controlling said corresponding wheel to prevent skidding of the latter; and control means for actuating said valve means including first means for generating a speed signal proportional to the anguLar velocity of said corresponding wheel, second means for differentiating said speed signal to generate an acceleration signal proportional to acceleration and deceleration of said corresponding wheel, means for generating first and second bias signals having predetermined values which are independent of fluctuations in said speed signal, said acceleration and deceleration signals, and the rate of deceleration of the vehicle, said means for generating said first and second bias signals being responsive to actuation of said valve means to change the values of said first and second bias signals by predetermined amounts which are independent of fluctuations in said speed and acceleration signals and of deceleration of said vehicle, switching means activating said valve means when said acceleration and deceleration signal drops below the value of the corresponding bias signal and terminating said valve means when said acceleration and deceleration signal exceeds the value of the then corresponding bias signal, bias means switchable from a first state to transmit said first bias signal to said switching means to a second state transmitting said second bias signal to said switching means, and means responsive to vehicle deceleration for switching said bias means between said first and second states.
 2. The invention of claim 1: said control means including mechanism generating a timer signal for a predetermined time period after actuation of said valve means, said means for generating said first and second bias signals being responsive to said timer signal for changing the value of said bias signals by said predetermined amounts.
 3. The invention of claim 1: said means responsive to the vehicle deceleration changing the first and second values of the bias signal by an equal predetermined amount which is independent of said speed and acceleration signals and of deceleration of said vehicle.
 4. The invention of claim 1: said means for generating first and second bias signal including a voltage dividing network comprising first resistance means connected in series with second resistance means, said first resistance means including a pair of resistors connected in parallel, said vehicle deceleration responsive switch means switching one of said pair of resistors into said network during vehicle braking on a low coefficient surface and switching said one of said pair of resistors from said circuit during vehicle braking on a high coefficient surface.
 5. The invention of claim 4: said control means including mechanism generating a timer signal for a predetermined time period after actuation of said valve means; said network including switch means controlling the value of said second resistance means, said switch means being responsive to said timer signal. 